The treatment of solid wastes has become a significant problem around the world. Since strict regulations prohibit the placement of hazardous wastes in landfills, combustion can become the principal and, probably, most attractive alternative for disposal of hazardous wastes. During combustion, the volume and mass of the waste are reduced to a fraction of its original size. Metals are not destroyed during combustion. A fraction of volatile toxic metals is emitted as vapours. Non-volatile toxic metal compounds are released with the solid ash and also require safe disposal. The chemical form, concentration and leachability of these compounds in ash depend on a number of factors including the composition of the waste and the treatment method used. This paper describes the method used to determine the optimum operating parameters of the pulp and paper sludge combustion process. Using these parameters, the characteristics of the combustion process as well as properties of the resultant ash are determined. The ash analysis includes morphology and metal fate. It was found that as combustion temperature rises, the solid residue changes from fibrous, powdered, sintered, molten, and finally stabilizes as a compact agglomerate (above 1400 K). An optimum set of operating conditions was established by design of cross-experiments (DCE) in order to study the process. The design of cross-experiments is a valuable experimental method for both scientific and practical utilization since it can shorten research time and save resources while optimizing operating parameters simultaneously.